#include <iostream>
#include <iomanip>
#include <cmath>

double r, R, h;

double sqr(double x) { return x * x; }

int main()
{
	std::cin >> r >> R >> h;
	double T = sqr(sqrt(sqr(R-r) + sqr(h)) - R) - sqr(r);
	double ans = (sqr(h) - T) / (2.0 * h);
	ans = std::min(ans, h / 2.0);
	std::cout <<  std::setprecision(8) << std::fixed << ans << '\n';
}
